For many years, electric gas discharges have been used in a variety of applications including etching, deposition, sterilization, functionalization, etc. Commonly, these devices require sub-atmospheric pressures necessitating costly pressure locks and vacuum systems. Dielectric barrier discharge (DBD) systems, however, can operate at, below, or even above atmospheric pressure. Most DBD systems have been driven by continuous wave, radio frequency (RF), power sources. In recent years, however, there has been increased use of pulsed power sources. In comparison with the RF DBD's, pulsed power DBD's, with their greater instantaneous powers, are able to achieve higher electron and reactive species densities together with higher electron energies leading to increased exposure dosage and decreased required processing time. In addition, the pulsed systems tend to be more stable and spatially uniform than the RF DBD's. Thus devices and techniques that lead to increases in power density without excessive gas heating, arcs, or narrow filamentary discharges are of considerable value.
As noted above, one useful application of plasma discharge systems is the use of the plasma for sterilization. U.S. patent application publication number US2004/0037736 A1 to Perruchot et al., which is hereby incorporated by reference, contains an extensive background treatment in the definition of sterilization and the various sterilization methods currently known and in use. As explained by Perruchot et al., the sterilization methods that use plasma discharge systems operate by creation of reactive species such as radicals of ionized and/or excited species. Various improvements on plasma discharge sterilization methods are further discussed in Perruchot.
Dielectric barrier discharges are commonly initiated by applying an alternating voltage across a gap between two electrodes where one or both of the electrodes are covered by a dielectric barrier material. DBD are non-equilibrium discharges which typically generate electrons with mean energies of a few eV in a non-thermal background gas. The dielectric barrier serves to separate the electrode from the plasma and is required to partially inhibit the direct flow of current between the two electrodes and distribute the discharge uniformly over the electrodes. The basic principle in most cases is to produce plasmas in which a majority of the electrical energy is used for the production of energetic electrons, rather than for gas heating, hence the plasma can enhance the gas phase chemistry without having to elevate the gas phase temperature (U. Kogelschatz, Plasma Chem. Plasma P. 23, 1 (2003)).
The applied voltage is commonly established in a DBD system using an RF source such that the peak voltage is slightly greater than the threshold voltage required to establish the glow discharge. As the voltage is increased above the threshold voltage a discharge occurs between the electrodes after a small time. The time difference between the time the threshold voltage is reached and the time the discharge initiates is referred to as the lag time. Typically the voltage rise time in RF systems is large compared to the lag time, and thus slowly increasing the voltage beyond this threshold after the discharge has been initiated will not increase the voltage potential across an electrode gap.
However, if the rise-times to achieve peak voltage of the voltage pulse are shorter than the lag time between the pulse crossing the threshold voltage and the onset of the discharge, the voltage pulse will continue to increase in value towards its peak value prior to the discharge. The application of a fast rising high voltage spike is thus said to create an “overvoltage” condition prior to discharge and has been predicted to produce among other things, higher energy electrons in the discharge (Bogdanov, J. Phys. D 37, 2987 (2004)). In the parent of the present application, an extreme overvoltage condition is advantageously utilized to produce a plasma in a DBD arrangement.
To increase the potential across an electrode gap a unipolar rapid rise time rectangular pulsed voltage source using two switching modules was developed by Liu and Neiger; however, the power source they disclosed produced only limited discharge currents of a few hundred milliamps (Liu, J. Phys. D 34, 1632 (2001)). Similar results were also reported by Spaan et al. with reported discharge currents up to five hundred milliamps (Spaan, Plasma Sources Sci. Technol. 9, 146 (2000)).
Pulse-forming networks were developed by Köhler to produce single rapid rise time voltage pulses (Köhler, Appl. Opt. 33, 3812 (1994)) and Blumlein configurations have been applied to generate rapid rise time, short pulse width voltage waveforms at frequencies ranging from 1 to 1000 Hz {Pouvesle, U.S. Pat. No. 5,651,045 (1997); Khacef, J. Phys. D. 35, 1491 (2002); Liu, IEEE Trans Plasma Sci. 33, 1182 (2005)}. The limited current outputs are partially solved by Blumlein configurations, but these produce pulses defined in part by the length of the transmission lines and the impedance across the transmission lines must be matched to the load to deliver maximum power. Such impedance matching makes it more difficult to scale the output to changes in the load such as electrode size, gap distance, dielectric, and/or gases solids or liquids in the gap as might be necessary for various applications of the plasma.